Data which is recorded on an optical disk is reproduced by irradiating the rotating optical disk with a light beam having a relatively weak constant light amount, and detecting reflected light which has been modulated by the optical disk.
On a read-only optical disk, information in the form of pits is recorded in a spiral manner, previously during manufacture of the optical disk. On the other hand, in the case of a rewritable optical disk, a method such as vapor deposition is used to deposit a film of recording material which allows for optical data recording/reproduction, on the surface of a base on which a track having spiral land or groove is formed. In the case where data is to be recorded on a rewritable optical disk, the optical disk is irradiated with a light beam whose light amount is modulated in accordance with the data to be recorded, thus causing local changes in the characteristics of the recording material film, whereby a data write is effected.
Note that the depth of the pits, the depth of the track, and the thickness of the recording material film are small relative to the thickness of the base of the optical disk. Therefore, any portion of the optical disk where data is recorded constitutes a two-dimensional surface, and may be referred to as a “signal surface” or an “information surface”. In the present specification, since such a signal surface (information surface) has a physical size along the depth direction, the term “information layer” will be employed, instead of “signal surface (information surface)”. An optical disk includes at least one such information layer. Note that one information layer may actually include a plurality of layers, e.g., a phase-change material layer and a reflective layer.
When reproducing data which is recorded on an optical disk, or recording data onto a recordable optical disk, it is necessary for a light beam to always retain a predetermined convergence state on a target track on the information layer. This requires “focus control” and “tracking control”. “Focus control” refers to controlling the position of an objective lens along a normal direction of the information surface (hereinafter may be referred to as the “depth direction of the substrate”) so that a focal point of the light beam (convergence point) is always positioned on the information layer. On the other hand, tracking control refers to controlling the position of an objective lens along a radial direction of the optical disk (hereinafter referred to as the “disk radial direction”) so that a spot of the light beam is positioned on a predetermined track.
As conventional high-density/large-capacity optical disks, optical disks such as DVD (Digital Versatile Disc)-ROMs, DVD-RAMs, DVD-RWs, DVD-Rs, DVD+RWs, and DVD+Rs have been put to practical use. In addition, CDs (Compact Discs) are still in use. Currently, next-generation optical disks which have a higher density and a larger capacity than those of the above optical disks are being developed and put to practical applications, e.g., Blu-ray Discs (BDs).
Such optical disks have various structures depending on their types. For example, they may differ in terms of the physical structure of tracks, track pitch, depth of the information layer (distance from the light-incident surface to the information layer of the optical disk), etc. In order to properly read data from or write data to a plurality of types of optical disks with such different physical structures, it is necessary to employ optical systems having numerical apertures (NA) which are in accordance with the types of optical disks, so as to irradiate the information layer of each optical disk with laser light of an appropriate wavelength.
FIG. 1 is a perspective view schematically showing the optical disk 200. For reference's sake, FIG. 1 shows an objective lens (converging lens) 220 and laser light 222 which has been converged by the objective lens 220. The laser light 222 is radiated onto the information layer via the light-incident face of the optical disk 200, thus forming a light beam spot on the information layer.
FIGS. 2(a), (b), and (c) schematically show general cross sections of a CD, a DVD, and a BD, respectively. Each optical disk shown in FIG. 2 has a surface (light-incident surface) 200a and a rear face (label face) 200b, and at least one information layer 214 therebetween. On the rear face 200b of the optical disk, a label layer 218 which contains a title and a printout of graphics is provided. Each optical disk has an overall thickness of 1.2 mm, with a diameter of 12 cm. For simplicity, protrusion/depression structures such as pits or grooves are not illustrated in the figures, and reflective layers and the like are also omitted from illustration.
As shown in FIG. 2(a), the information layer 214 of a CD is positioned at a depth of about 1.2 mm from the surface 200a. In order to read data from the information layer 214 of a CD, it is necessary to converge near-infrared laser (wavelength: 785 nm), which is controlled so as to have a focal point positioned on the information layer 214. The numerical aperture (NA) of an objective lens which is used for converging the laser light is about 0.5.
As shown in FIG. 2(b), the information layer 214 of a DVD is positioned at a depth of about 0.6 mm from the surface 200a. In an actual DVD, two substrates having a thickness of about 0.6 mm are attached together via an adhesion layer. In the case of an optical disk having two information layers 214, the distances from the surface 200a to the information layers 214 are about 0.57 mm and about 0.63 mm, i.e., they are close. Therefore, regardless of the number of information layers 214, only one information layer 214 is described illustrated in the figure. In order to read data from or write data to the information layer 214 of a DVD, it is necessary to converge red laser (wavelength: 660 nm), which is controlled so as to have a focal point positioned on the information layer 214. The numerical aperture (NA) of an objective lens which is used for converging the laser light is about 0.6.
As shown in FIG. 2(c), a BD includes a thin cover layer (transparent layer) having a thickness 100 μm which is provided on the side of the surface 200a, and the information layer 214 is positioned at a depth of about 0.1 mm from the surface 200a. In order to data from the information layer 214 of a BD, it is necessary to converge blue-violet laser (wavelength: 405 nm), which is controlled so as to have a focal point positioned on the information layer 214. The numerical aperture (NA) of an objective lens which is used for converging the laser light is 0.85.
FIG. 3(a) is a schematic diagram showing spherical aberration being caused by the objective lens 220 when entered by parallel light. FIG. 3(b) is a schematic diagram showing how the spherical aberration is corrected by allowing divergent light to enter the same objective lens 220.
The magnitude of such a spherical aberration also changes depending on the thickness of a portion existing from the surface of the optical disk to the information layer (which may also be referred to as “base thickness”), that is, “depth of the information layer”. FIG. 4 shows a light beam which has entered the optical disk 200 being converged on the information layer 214. Since the rays which compose the light beam are refracted at the surface of the optical disk 200, the convergence state of the light beam will vary in accordance with the refractive index and thickness of the portion existing from the surface of the optical disk 200 to the information layer 214. Therefore, even in the case where no spherical aberration occurs for a specific optical disk, spherical aberration may occur for an optical disk having a different base thickness.
As mentioned above, optical disks with various structures are available on the market, and there is desired an ability to support such a plurality of types of optical disks with a single apparatus. Such an optical disk apparatus will need to have a construction in which a light beam is selected as appropriate from among a plurality of light beams of different wavelengths, and irradiates an optical disk with a reduced spherical aberration.
Next, with reference to FIG. 20, a conventional example of an optical disk apparatus supporting a plurality of types of optical disks is described. For simplicity, FIG. 20 only illustrates the construction on the forward path side (i.e., starting from a light source and heading toward the disk surface), and the construction on the return path side (i.e., starting from the disk surface and heading toward a photodetector) is not illustrated.
The optical disk apparatus of FIG. 20 includes three light sources 1B, 1R, and 1I. Blue light (e.g., wavelength: 0.405 μm) which is emitted from the light source 1B, such as a blue-light emitting semiconductor laser, is reflected by a dichroic mirror prism 2 (which reflects blue light and transmits wavelengths longer than blue), and travels through a collimating lens 3 so as to be converted into plane waves 4B (a so-called infinite system). The plane waves 4B receive aperture restriction by a color-selective aperture filter 5 so as to have a numerical aperture corresponding to NA 0.85, and thereafter is transmitted through an objective lens 6 (e.g., NA 0.85 or more) to enter an optical disk. The light having entered an optical disk base 7B, having a thickness of 0.1 mm, is converged on an information layer 8B which is formed on the rear face of the base 7B. For simplicity, FIG. 20 simultaneously illustrates different optical disk bases 7B, 7R, and 7I and information layers 8B, 8R, and 8I, corresponding to the three light sources 1B, 1R, and 1I. In actuality, however, one optical disk that corresponds to one of the light sources is to be mounted in the optical disk apparatus.
On the other hand, red light (e.g., wavelength: 0.660 μm) which is emitted from the light source 1R, such as a red-light emitting semiconductor laser, is transmitted through a dichroic mirror prism 9 (which reflects infrared light and transmits wavelengths shorter than infrared light) and the dichroic mirror prism 2, and travels through the collimating lens 3 so as to be converted into divergent spherical waves 4R (a so-called finite system). The spherical waves 4R receive aperture restriction by the color-selective aperture filter 5 so as to have a numerical aperture corresponding to NA 0.6, and thereafter is transmitted through the objective lens 6 to enter an optical disk. The light having entered the optical disk base 7R, having a thickness of 0.6 mm, is converged on the information layer 8R which is formed on the rear face of the base 7R.
Furthermore, infrared light (e.g., wavelength: 0.790 μm) which is emitted from the light source 1I, such as an infrared-light emitting semiconductor laser, is reflected by the dichroic mirror prism 9 and transmitted through the dichroic mirror prism 2, and thereafter travels through the collimating lens 3 so as to be converted into divergent spherical waves 4I (a so-called finite system). The spherical waves 4I receive aperture restriction by the color-selective aperture filter 5 so as to have a numerical aperture corresponding to NA 0.5, and thereafter travels through the objective lens 6 to enter an optical disk. The light having entered the optical disk base 7I, having a thickness of 1.2 mm, is converged on the information layer 8I which is formed on the rear face of the base 7I.
The objective lens 6 is designed so that, with respect to the optical disk base 7B having a thickness of 0.1 mm, the light entering an infinite system at the wavelength of 0.405 μm will be converged with no aberration. Therefore, if light enters the infinite system at the wavelength of 0.660 μm or the wavelength of 0.790 μm with respect to the optical disk base 7R having a thickness of 0.6 mm or the optical disk base 7I having a thickness of 1.2 mm, respectively, a large spherical aberration will occur. The occurrence of spherical aberration is ascribable not only to differences in base thicknesses, but also refractive index dispersion in the objective lens, and spherical aberration in the disk base.
In the conventional example of FIG. 20, by bringing the positions of the light sources 1R and 1I closer to the collimating lens 3 along an optical axis L, the light having been transmitted through the collimating lens 3 is allowed to turn into divergent spherical waves 4R and 4I. By thus allowing the light sources 1R and 1I to function as light sources of a finite system, the aforementioned spherical aberration is cancelled.
[Non-Patent Document 1] Nikkei Electronics (Sep. 27, 2004 issue) P101-121